Older nobleman that is king (7)
I believe the answer is:
earlier
'older' is the definition.
('early' can be similar in meaning to 'old')
'nobleman that is king' is the wordplay.
'nobleman' becomes 'earl' (member of the nobility).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'king' becomes 'R' (abbreviation of Latin rex).
'earl'+'ie'+'r'='EARLIER'
(Other definitions for earlier that I've seen before include "Prior" , "in the past" , "Older" , "Former" , "At a previous time" .)